This invention relates to electrically conductive lignocellulosic particle boards. It pertains particularly to electrically conductive lignocellulosic particle boards of use in establishing static-free environments for the manufacture and operation of microcomputers and associated and/or related components and equipment, as well as for the control of static electrical discharge problems generally.
Important locations which must be protected from static electrical accumulation and discharge include any involved with the production or use of volatile inflammable liquids and gases. Hospital operating rooms are an example. Quoting from Sections 3--3.6.1.2 and 1.3 of the Americal National Standards Institute/National Fire Protection Standard 99 for Health Care Facilities:
1.2 "The prevention of the accumulation of static charges revolves about a number of safeguards which shall be complied with in flammable anesthetizing locations, in corridors and passageways adjacent thereto, in rooms connecting directly to anesthetizing locations, such as scrub rooms and sterilizing rooms, and in storage locations for flammable anesthetics located win an operating suite."
1.3 "The method employed to prevent such accumulation include the installation of conductive floor (see 3--3.6.2), the maintenance of the relative humidity at 50 percent at least, and the use of certain items of conductive equipment, accessories and wearing apparel."
It is well known that the operating voltages of microcomputers and related equipment are now so low that the discharge of static electricity in the vicinity of the equipment may easily erase the memory or even damage the microcircuits of the equipment, to the great dismay of both equipment manufacturers and users. As a result, every effort must be made to produce and operate the equipment in a static-free environment, including the provision of electrically-grounded furniture, floors and walls in the equipment production facilities and user installations.
In such rooms and booths, it is common practice to use lignocellulosic particle boards, particularly wood particle boards, having thicknesses in the range of from one-eighth to two inches in the manufacture of the flooring, wall panels, cabinets and furniture.
Such particle boards are conventionally made by comminuting wood or other lignocellulose to the form of small pieces, blending mixtures of the small pieces with a suitable thermosetting adhesive, dry-forming the resulting mixture into a mat, and hot-pressing the mat (in a platen press) to the desired density and thickness.
Varieties of particle board manufactured in this manner are chipboard, flakeboard, and strand (or sliver) board, which is used in the manufacture of oriented "strand boards". Closely related fiber-based products are conventionally termed low density, medium density and high density fiberboards.
Following are product definitions from various recognized sources;
Particle board is a generic term for a panel manufactured from lignocellulosic materials (usually wood) primarily in the form of discrete pieces or particles, as distinguished from fibers, combined with a synthetic resin or other suitable binder and bonded together under heat and pressure in a hot press by a process in which entire interparticle bond is created by the added binder and to which other materials may have been added during manufacture to improve certain properties. (ASTM D-1554-86, Standard Definitions of Terms Relating to Wood-base Fiber and Particle Panel Materials).
Chipboard is a particle panel composed of small pieces of wood chopped off a larger piece by axe-like cuts as in a chipper of the paper industry, or produced by mechanical hogs, hammermills, etc. (ASTM D 1554-86). The term chipboard is frequently used to identify particle board made from mill waste raw materials such as planer shavings, hogged mill ends and sawdust.
Flakeboard is a particle panel composed of flakes. Flakes are small flat wood particles of predetermined dimensions, substantially uniform in thickness, with fiber direction essentially in the plane of the flake; in overall character resembling a small piece of veneer. It is produced by special equipment for use in the manufacture of flakeboard. (Wood Handbook: Wood as an Engineering Material. USDA Agriculture Handbook 72).
Oriented Strandboard is a type of particle panel product composed of strand-type flakes which are purposely aligned in directions which make a panel stronger, stiffer and with improved dimensional properties in the alignment directions, as compared with a panel with random flake orientation. (Wood Handbook: Wood as an Engineering Material (USDA Agriculture Handbook 72).
Fiberboard is a dry formed panel product manufactured from lignocellulosic fibers combined with a synthetic resin or other suitable binder. The fibers are slender, threadlike elements resulting from the chemical or mechanical defiberization of woody materials, with or without steam refining. In the manufacture of medium density fiber board, the panels are compressed to a density of 0.50 to 0.80 gm./cm cm, in a hot press by a process in which substantially the entire interfiber bond is created by the added binder. Other materials may have been added during manufacture to improve certain properties. (American National Standard, Medium Density Fiberboard for Interior Use. ANSI A208.2-1980).
All of these products are to be distinguished from paper products based on chemically pulped wood fibers in that (1) they comprise lignocellulose products rather than cellulosic products from which the lignin has been removed and (2) they have appreciable thickness (herein from 1/8-inch to 2-inches) and hence have appreciable structural strength. As noted, they have primary application as flooring, wallboard, cabinets and furniture components.
Electrically conductive paper products are known. As illustrated in Davenport et al. U.S. Pat. No. 2,328,198, they are manufactured by including electrically conductive carbon particles in the papermaking furnish which is converted to the finished paper sheet. Alternatively, as illustrated in Ohlbach, U.S. Pat. No. 4,160,503, they may be manufactured by sprinkling electrically conductive carbon particles on the surfaces of paper or paperboard sheets. The object is to provide on or in the sheet a continuous, electrically conducting path which in use leads to ground any electric charges which may develop on the sheet.
However, particle board products having this desired capacity have not heretofore been developed. Manifold problems attend the manufacture of electrically conducting boards using comminuted lignocellulose as a primary raw material. As will be shown hereinafter, certain forms of lignocellulose of reduced size, for example refined wood fibers, are not operable. Similarly, all electrically conductive materials are not operable, for example electrically conductive inorganic salts, or even particles of electrically conducting metals or metal oxides such as aluminum oxide and iron oxide. Still further, although finely divided carbon particles are operable, they function successfully only within carefully defined parameters.
Accordingly it is the general object of the present invention to provide a particle board product useful in the described and related applications.
A further important object of the present invention is the provision of such a product at various controllable levels of conductivity produced by procedures which are compatible with current conventional particle board manufacturing operations.
We have discovered that electrically conductive particle boards of the class described may be manufactured practically and economically at various controllable levels of conductivity by procedures which are compatible with current particle board manufacturing procedures by dryforming and subsequently hot-consolidating a mat having the following composition, in % by weight, dry solids basis:
______________________________________ Finely divided, electrically conductive carbon particles 0.2-5 particle board adhesive 1.5-15 lignocellulose particles balance ______________________________________
In the foregoing, the lignocellulose particles comprise at least one member of the group consisting of lignocellulose chips, lignocellulose flakes, and lignocellulose strands.
In addition to the above, minor proportions of process fines and of additives such as pigments, fire proofing agents, rot proofing agents, wax emulsions and the like may be employed as desirable or necessary for particular purposes.
To secure effective, complete coverage of the foregoing lignocellulosic components, the carbon particles used with them must have surface areas of at least 20 square meters per gram.
When these conditions are fulfilled, electrically conductive particle products may be fabricated from wood and other forms of lignocellulose in the form of boards or billets having the desired properties.